Method of nitriding metals



Patented May 2, 1933 UNITED STATES AUGUSTUS B. KINZEI L, 0F BEECHHURST,

ELECTRO METALLURGICAL COMPANY,

PATENT OFFICE METHOD OF NITRIDING METALS No Drawing. Application filed May 26,

My invention relates to the production of nitrided layers on metals by heating the metals in the presence of suitable nitrogencontaining agents, such as ammonia. It has particular reference t w an improved method of forming such nitri ed layers. v

The object of my invention is to provide a means by which the action of nitriding gases on metal articles is accelerated and intensified, so as to reduce the time required to produce a case of given hardness or thickness.

The case hardening of alloys is useful as a means of forming hard wear-resisting surfaces on articles which are to be subject to abrasion. In order to form adherent and non-brittle cases, and to prevent distortion of the-articles undergoing treatment, it is customary to carry out the production of nitride cases at low temperatures, usually of the order of4-.60 C. to 580. To produce satisfactory cases at these low temperatures it has been found advisable to alloy with the material to be treated .small amounts of a promoting agent such as aluminum or vanadium. The procedure customarily followed consists, essentially, in

heating the alloyed articles in a mufiie or similar externally-fired furnace in the pres ence of a nitriding agent. When this procedure is followed a heating period offrom 12 to 20 hours is required to form a useful case, the time of heating increasing with a decrease in temperature employed. Such prolonged heating is expensive, and at the present time forms a large part of the total cost of the nitriding process; It is evident that a saving in the time of heating required to produce a case of the desired hardness and thickness would result 'in a material reduction in the'cost of nitriding.

I have discovered that a remarkable increase in the speed of production of a nitride case by contact with a nitrid-ing gas occurs when the customary process is so modified that the heating is done directly by the action of electric currents electromagnetically induced in, or in the surface of,- the articles; under treatment, instead of by the indirect/heating heretofore used. When 1931. Serial No. 540,207.

the article to be nitrid-ed is thus heated by induction, the rate of hardening is increased to a surprising degree, as will be demonstrated below.

According to my invention, the heating may be done by any suitable means whereby electroniagnetically induced electric currents are produced in the articles of suflicient magnitude to cause a rise of temperature of the surface of those articles to the temperature at which the nitriding reactions are to be carried out. It should be noted that if high voltages are used in induction heating,

the electromagnetically induced currents are largely restricted to the surface of the article. Similarly, the nitridii'ig'gas may be any suitable gas of this description, and may be applied to the article by any feasible means or method.

(In the preferred form of my invention /the articles are placed in a chamber containing a gaseous nitriding agent and surrounded by a coil carrying high-frequency electric currents. The chamber is preferably so constructed and equipped that a continuous slow flow of the nitriding gas through the chamber may be used. By this arrangement, high-frequency currents are induced in the metal articles, whereby the surface temperature of those articles is raised to the desired reaction temperature. Under these conditions a definite case is produced in materially shorter time than under conditions heretofore employed in the art. Forl example: Several pieces of steel, alloyed with small amounts of chromium and aluminum, were heated for four hours at a temperature of 460 C. by means of electric currents of high frequency induced in the test pieces in an electric induction furnace. During this heating period a slow stream of ammonia gas was passed over the steel pieces. At the end of this treating period thesamples showed a definite, hard and adherent cases The average hardness of the samples before this treatment was about 98 Rockwell, while'their average hardness after this treatment was about 115 Rockwell, sufliciently hard to scratch glass with ease; the average depth of case produced was from ten to fifteen th'ousandths of an inch. Pieces of similarly alloyed steel, heated in a stream of ammoniafor four hours at the same temperature in a muflle furnace according to the commonly known procedure, showed no appreciable hardening, as was to be expected from common experience in the art.

The particular furnace used in the above test is a standard 20 k. 'w. condenser-type high-frequency induction furnace utilizing a frequency of approximately 10,000 cycles per second. It will be understood that the use of a standard furnace, or of this stated frequency, is not essential to secure improvement in the art of nitriding as now practiced with an externally fired furnace. Any practical means of directly heating the articles by electric currents electromagnetical- 1y inducedtherein, aswell as the use of any frequency that will cause a rise of temperature of the surface of the articles to the reaction temperature, gives the improvement sought in this invention. It is to be pointed out however that the best results are to be secured by the use of higher frequencies, such as those of the' order used in the examples given above.

It will be evident that numerous modifications of the method as described above may be made without-departing from the spirit of my invention as disclosed in this specification and the appended claims.

I claim: y a

1. The method of nitriding metal articles which comprises heating the articles in the presence of anit-riding agent by means of high-frequency electric currents electromagnetically induced in said articles.

2. The method of nitriding metal articles which comprises heating the articles in the presence of a nitriding agent by means of high-frequency electric currents induced in said articles.

3. The method of nitriding ferrous articles which comprises heating the articles in the presence of ammonia by means of highfrequency electric currents induced in said articles.

4. The methodof nitriding ferrous articles which comprises heating the articles in the presence of ammonia by means of highfrequency electric currents electromagnetically induced in the surface of said articles.

5. The method of nitriding ferrous articles containing nitride-promoting agents such as chromium, aluminum, or vanadium, which comprises heating the articles in the presence of ammonia by means of'electric currents of a frequency of the order of 10,- 000 cycles per second induced in said articles.

In testimony whereofi'I aflix my signature. AUGUSTUS B. KINZEL. 

